The purpose of this study is to investigate the safety and tolerability (the ability of the body to accept the medication) of several different doses of IB-367 administered via a nebulizer to patients with moderate cystic fibrosis lung disease. It is hoped that this study will help select the appropriate dose for further studies of IB-367 in patients with cystic fibrosis. Patients in this study will receive either IB-367 or a placebo (an inactive substance). The assignment to the treatment group is done on a random basis, and is determined before the study begins.